The phase-locked loop (PLL) is a useful building block available from several manufacturers as a single integrated circuit. A phase-locked loop PLL typically contains a phase detector PFD, an amplifier A.sub.V, a low pass filter LPF (both contained in a low pass filter means LPFM) and a voltage-control oscillator VCO as is schematically shown in FIG. 1a. In a phase-locked loop a mixture of digital and analogue techniques are combined in one package. The phase-locked loop finds broad applications in tone decoding, demodulation of AM and FM signals, frequency multiplication, frequency synthesis, pulse synchronization of signals from noisy sources, e.g. magnetic tape, and regeneration of "clean" signals.
The basic operation of the phase-locked loop is as follows. The phase detector PFD is a device that compares two input frequencies, i.e. the predetermined frequency f2 of an output signal from the voltage controlled oscillator and a predetermined reference frequency f1 of a reference signal f1. The phase detector generates an output that is a measure of the phase difference between the two signals f1, f2 (if, for example, they differ in frequency, it gives a periodic output at the difference frequency). If f1 is not equal to f2 the phase deviation signals, after being filtered and amplified in the low pass filter means LPFM, causes the voltage-controlled oscillator frequency to deviate in the direction of f1. If the operating conditions are correctly set, the voltage-controlled oscillator will quickly "lock" its output frequency f2 to the reference frequency f1, maintaining a fixed phase relationship with the input signal.
The generated control voltage input to the VCO is a measure of the output frequency f2. The VCO output is a locally generated frequency, in the simplest case equal to f1 thus providing a clean replica of f1, which may itself be noisy. The wave forms of f1, f2 are not restricted to any particular waveform, i.e. the VCO output signal can be a triangle wave, sign wave or any other wave. Therefore, the phase locked loop PLL provides an easy technique, for example to generate a sign wave locked to a train of input pulses.
Since the phase detector PFD compares the phase (or the frequency) of the reference frequency f1 with the predetermined frequency f2 output by the voltage controlled oscillator the phase deviation signal S is a signal which--dependent on the tracking behavior of the phase-locked loop PLL--assumes values according to the phase/frequency deviation between f1 and f2.